In the case of many functional parts, for example spur gear wheels or bearing journals, in particular of journal crosses (=spiders) of Cardan joints, there is the general problem that it is intended to achieve a functional contour that is made as long as possible with respect to the axial direction in relation to the overall size by a production process that is as simple as possible. For example, in the case of a spur gear wheel, the toothing over a portion of the axial extent (=functional portion) of the spur gear wheel over which there are teeth with the size and shape that is at least substantially intended forms such a functional contour. In the case of a bearing journal, the lateral surface of the bearing journal over the portion of the axial extent (=functional portion) of the bearing journal in which the lateral surface is at least substantially cylindrically formed is the functional contour.
Furthermore, the functional contour is intended to have a high degree of dimensional accuracy over its axial extent, in particular with regard to a distance from the longitudinal central axis of at least one characterizing element of the functional contour remaining as constant as possible over the axial extent of the functional contour. Bearing journals are intended to have a form that is as exactly cylindrical as possible at least over a functional portion of their axial extent, that is to say the lateral surface of the bearing journal is intended to correspond at least over this region of the axial extent of the bearing journal as exactly as possible to a shell of a cylinder, in particular a shell of a circular cylinder, in order to form a good running surface for rolling bodies, in particular needles. In order to achieve a good tooth engagement, which can also transfer high loads, in the case of spur gear wheels the distance of the profile reference line, that is the line along which the tooth thickness is equal to the gap width, from the longitudinal central axis should be as constant as possible over the axial extent of the toothing.
Spur gear wheels are also known by the term spur gears or cylindrical gears. In the standard DIN 3960 of March 1987, spur gear wheels that form functional parts of the type mentioned at the beginning are described. The term “profile reference line” is defined for a spur gear wheel, as disclosed in particular by subclause 3.2 and FIG. 1 of DIN 867 from February 1986. As mentioned, the toothing forms a functional contour that extends over a functional portion of the spur gear wheel. The functional portion may comprise here part of the axial extent of the spur gear wheel or extend over the entire axial extent of the spur gear wheel. In the case of a straight-toothed spur gear wheel, it is intended that the tooth crests extend parallel to the longitudinal central axis, while in the case of helically toothed spur gear wheels they form an angle unequal to zero degrees with said axis, the distance from the longitudinal central axis in each case being intended to remain constant.
Spur gear wheels are conventionally produced by cold extrusion. The production of a spur gear wheel by cold extrusion is disclosed for example by EP 0 560 010 A1. The two dies (=mould halves) have aligned channels that run at right angles to the parting plane between the dies and into which a blank is inserted. Extrusion punches are moved through the channels against the cylindrical blank from both sides, whereby a material flow of the material of the blank takes place into radial clearances in the channels of the dies. The material flow forming the teeth consequently takes place transversely in relation to the direction in which the extrusion punches are moved. Cold extrusion, in particular transverse extrusion, is likewise conventionally used for producing journal crosses (=spiders) with bearing journals, the longitudinal central axes of the bearing journals lying in a plane and the longitudinal central axes of successive bearing journals respectively being at right angles to one another, or tripods, in which three bearing journals lie in one plane and the longitudinal central axes of said journals respectively form an angle with one another of 120°. The production of a journal cross by cold extrusion, in particular transverse extrusion, is disclosed for example by DE 2819167 A1. The two dies (=mould halves) have in turn aligned channels that run at right angles to the parting plane between the dies, are cylindrically formed here and into which a blank is inserted. Extrusion punches are moved through the cylindrical channels against the cylindrical blank from both sides, whereby a material flow of the material of the blank takes place into clearances in the dies that adjoin the cylindrical channels in the region of the parting plane and have the form of the journals to be formed. The material flow forming the journals consequently takes place transversely in relation to the direction in which the extrusion punches are moved. Here, the bearing journals form functional parts of the journal cross.
In the production of functional parts of the type mentioned at the beginning that have a functional contour by cold extrusion, at least part of the end face and/or a region of the outer surface of the functional part that lies at the transition from the lateral surface to the end face is formed by a free-form surface at a free end of the functional part. In other words, in the region of the free-form surface, the material of the functional part does not run up against a wall of the extrusion tool, that is to say a wall of one of the dies or one of the extrusion punches, during the extrusion. Such a free-form surface forms an outwardly convex elevation of the functional part. Generally, the transition between this free-form surface and the functional contour bounds the functional portion of the functional part.
In order to improve the length of the functional portion of a spur gear wheel, in EP 0 560 010 A1 a complex tooling technique and movement is proposed. To improve the dimensional stability of the functional contour, various methods for re-working the component are known in the prior art. For example, for this purpose a grinding operation or some other machining method may be carried out. Machining operations have the disadvantage, however, that they are time- and cost-intensive, and so such machining operations should be avoided as far as possible. It is known from EP 0891825 B1 in the case of bearing journals, here in particular tripods, to carry out a rolling operation for subsequent treatment after the extrusion. If desired, grooves running around the bearing journal may at the same time be introduced into the lateral surface of the bearing journal in this rolling operation.